Chapter 9 - Periodicity

Question 1

Does the atomic radius increase or decrease across period 3 and why?

Answer 1

• Increase

- Bc of increased nuclear charge across the period, which draws outer electrons closer to the nucleus

Question 2

Describe and explain the trend in the melting points from Na to Ar

Answer 2

-Metallic elements increase
o From Na to Al the n. of outer shell electrons increases * greater attraction between posistive ions and delocalised e-
o Size of ions also increases leading to increased attraction

-Melting point of silicon is very high
o Giant covalent structure
o A lot of energy required to break many strong covalent bond

• Melting p of Phosphorus, Sulfur, chlorine and Ar low
  o Simple covalent molecules * little energy required to overcome the weak van der Waal forces

Question 3

Why does the melting point increase from phosphorus to sulfur?

Answer 3

Because sulfur (S8) is a bigger molecule than P4, therefore has more electrons * more van der Waal forces need to be over come * more energy

Question 4

Describe the trend of first ionisation energy across a period in general

Answer 4

Increases

• Increased nuclear charge w no increased shielding
• Atomic radius decreases
• Outer electron is closer to the nucleus w greater nuclear charge * more energy required

Question 5

Why are the first ionisation energies lower than expected in group 3?

Answer 5

-Due to division of the energy levels into sub-shells
-E.g. Al 1s2 2s2 2p6 3s2 3p1
3p1 e- is further from the nucleus and has additional shielding from 3s2 inner electrons so requires less energy

Question 6

Do the atoms of group 6 show a lower or higher than expected ionisation energy?

Answer 6

• Lower
• Bc of the pairing of electrons in p sub-shell

3p4 electron config of sulfur
1s2 2s2 2p6 3s3 3p4

Means that 2 electrons are paired in the p sub-shell (should be 6 electrons), additional repulsion betweeen these 2 electrons lowers the energy required to remove one of the electrons

Question 7

Suggest the trench in atomic radius down group 2

Answer 7

• Increases
• Shielding increases as the n. of electrons increases per molecule
• Therefore outer shell e- is held less tightly
• Electrons also provide extra repulsion and outer e- becomes further away from attractive force of nucleus